First ultrasound

I had my first ultrasound done today and they had asked the first day of my LMP. I told them the day I began bleeding since the last period was a miscarriage. So they went with that. Which would make me 7 weeks and 1 day. However according to when I ovulated my possible day of conception makes me 6 wks 4 days which is what the baby measured at. But they didn't change my due date? Had anyone else had this happen after miscarriage and conceiving right after?